For calculation of the cyclic and emergency current rating of cables acc. IEC 60853, the multi-core cable is replaced by an equivalent single-core construction dissipating the same total conductor losses. The equivalent single-core conductor has a diameter as calculated by the formula below where $D_i$ is the value of diameter over insulation and $T_1$ the thermal resistance for the multi-core cable.
The actual conductors are considered to be completely inside the diameter of the equivalent single-core conductor, the remainder of the equivalent conductor being occupied by insulation.
| $d_c$ | single-core cables |
| $\sqrt{2} d_c$ | two-core cables, round conductors |
| $D_{it} e^{\frac{-2\pi T_1}{n_c \rho_i}}$ | three-core cables, round conductors |
| $d_x$ | multi-core cables, sector-shaped conductors |
